The invention relates to a lead-and barium-free crystal glass for producing high-grade glass-objects with a refractive index higher than 1.52 and a density of at least 2.45 g/cc manually or mechanically.
Lead and barium are toxic in very small amounts. They are contained in conventional crystal glass and can be leached out after a short time from objects manufactured from such crystal glass and can reach the human organism. For this reason, there is increased interest in lead-and barium-free crystal glass. In order to replace the lead and barium, which are present in crystal glass in the form of PbO and BaO, K2O and/or ZnO are added in increased amounts to known lead-and barium-free crystal glasses, as described, for example, in the DE 43 03 474 C2. By these means, a lead- and barium-free crystal glass can be melted which, on the one hand, essentially has the physical and chemical properties of lead- and barium-containing crystal glass and on the other, complies with the legal requirements for lead- and barium-free crystal glass, namely, a refractive index higher than 1.52 and a density of at least 2.45 g/cc. Even though the physical and chemical properties of known lead- and barium-free crystal glasses essentially correspond to those of previously known lead- and barium-containing crystal glasses, this applies also to the processing conditions, that is, the melting and processing temperatures of now known lead- and barium-free crystal glasses also lie within the range of previously known xe2x80x9ccustomaryxe2x80x9d crystal glasses. As a rule, the melting temperature is about 1450xc2x0 C. and the -viscosity hu=104 dPas at temperatures above 1000xc2x0 C. Accordingly, on the one hand, a necessary minimum viscosity must be reached so that the processability is adequate and, on the other, the glass temperatures should not be too high, with regard to the strain on the processing machines.
Furthermore, DE 693 20 994 T2 is named as state-of-the-art. From this publication, a lead- and barium-free crystal glass is known with a refractive index higher than 1.52. This glass is intended for hand-made or machine-made glassware for daily use with a high gloss and a high light transparency. The glass compositions, described there, are to be processed essentially under the processing parameters described above.
It is an object of the invention to indicate a lead- and barium-free crystal glass, which can be processed in an improved manner.
This can be accomplished with a lead- and barium-free crystal glass of the type named above, which is characterized by a composition in weight percent of
as well as at least two components from the group consisting of Er2O3, Nd2O3, CeO2, CoO, Pr2O3, SeO, NiO and MnO and having a moisture content of 0.025 to 0.07% by weight.
The inventive crystal glass with a higher water content ranging from 0.025 to 0.007% by weight has a viscosity, which is different appreciably from that of crystal glass with a lower water content. The water, which is incorporated in the glass structure in the form of silanol groups, causes the glass to have a viscosity at a particular temperature, which is lower than that of crystal glasses, which have a lesser water content. The glass thus becomes more fluid. This is associated with the appreciable advantage that there is increased heat radiation during the molding process of the glass that is to be processed, that is, the heat is dissipated more rapidly in the glass mold. Moreover, a more rapid cooling may be observed. Because of the lower viscosity, the machines can run more quickly, since the processable glass can be transported more rapidly through the machine. In other words, while retaining the previous processing temperatures, the glass can be processed more efficiently because of the lower viscosity achievable due to the increased water content. Alternatively, it is also possible to lower the processing temperature of the glass, since the usual processing viscosity of h=104 dPas is achieved at clearly lower temperatures. The temperature difference DT greater than  greater than xxc2x710xc2x0 C. depends, on the one hand, on the water content and, on the other, on the glass composition selected. In turn, this has an advantage is effect on the strain on the machine and, with that, on the service life.
The increase in the water content of the glass, proposed pursuant to the invention, can be achieved, on the one hand, by using oxygen heating for melting the glass composition, especially in conjunction with water-containing raw materials. As xe2x80x9cmoistxe2x80x9d raw materials, NaOH, Ca(OH)2, Al(OH)3, Na2B4O7xH2O, etc. and the like may be used. An increase in the water content can be attained simply in conjunction with the water formed during the combustion of oxygen-rich heating gases. Alternatively, it is possible to increase the water content solely by oxygen heating by conducting the heating process appropriately. Finally, the possibility exists when an electric vat is used for melting the glass composition, to blow steam into the vat region and moisten the furnace atmosphere in this way. The water penetrates into the melt and is incorporated in it.
Of the glass components named above, SiO2 acts as a network forming agent. TiO2 has a chromophoric effect with regard to an Fexe2x80x94Ti oxide complex that is formed and, in contrast to SiO2, increases the refractive index.
Al2O3 increases the chemical stability and stabilizes the network. Furthermore, it decreases the corrosion of the vat.
CaO, MgO and SrO increase the chemical stability and affect the viscosity of the glass. CaO leads to a xe2x80x9cshortxe2x80x9d glass. On the other hand, MgO and SrO bring about the formation of a xe2x80x9clongxe2x80x9d glass.
ZnO acts as an intermediate oxide and brings about an increase in the refractive index. Li2O, Na2O and K2O have the function of a network converter (flux). Of these, Li2O brings about an increase in the refractive index.
Sb2O3 or AS2O3 are added as refining agents to reduce bubble and schlieren formation. The two are interchangeable.
B2O3 is a strong flux, which increases the chemical stability.
Finally, sulfate (SO42xe2x88x92) is present. It also is a refining agent. In addition, a small amount of fluoride (Fxe2x88x92) is present, which decolorizes and interferes with the formation of the above-named Fexe2x80x94Ti oxide complex. However, the Fxe2x88x92 content should remain small in view of the possible formation of HF gas in the vat. Moreover, the inventive glass contains at least two components selected from the group consisting of, in percent by weight
these compounds being decolorizing agents. The objective is to obtain a clear and transparent glass. The role of praseodymium trioxide, in particular, should be mentioned. Because of its fluorescing properties, praseodymium has a slightly violet coloration and is normally added as a complementary color to Fe ions, which lead to a green coloration. In addition, if the praseodymium content is sufficiently high, its fluorescing property enables the glass as a whole to become slightly fluorescing. This contributes to the optics, since the glass then has a slight afterglow effect.
It is particularly appropriate if the water content is between 0.035 and 0.06% by weight.
It has also proven to be advantageous if the crystal glass contains
0.001-4.0% by weight of La2O3 and
0.001-3.0% by weight of SnO.
La2O3 brings about an increase in the refractive index. SnO acts as a refining agent and also reduces bubble and schlieren formation.
It has furthermore proven to be advantageous if the content of glass components, which function as decolorizing agents and of which two and preferably more are present for the production of a transparent, crystal clear glass, fall within the ranges of weight percent given below: